1. Field of the Invention
The present invention relates generally to an optical head device and an optical recording and reproducing apparatus that can record information on or reproduce recorded information from optical information recording media having different optical characteristics and recording densities from one another.
2. Related Background Art
Currently, the optical information recording media that have been in wide use for recording music, image data, or data files include, for example, CD-standardized disks such as a compact disk (hereinafter referred to as “CD”), a CD-read-only memory (CD-ROM), a recordable CD (CD-R) and a rewritable CD (CD-RW), and DVD-standardized disks such as a digital video disk (hereinafter referred to as “DVD”), a DVD-read-only memory (DVD-ROM), a recordable DVD (DVD-R), a rewritable DVD (DVD-RW, DVD+RW) and a DVD-random access memory (DVD-RAM). Light sources to be used for carrying out recording or reproduction with respect to a CD-standardized disk and a DVD-standardized disk having a larger recording capacity than that of the CD-standardized disk are a near-infrared semiconductor laser having an emission wavelength of 780 nm to 820 nm and a red semiconductor laser having an emission wavelength of 630 nm to 690 nm, respectively. Recently, a single apparatus has been requested to allow recording and reproduction to be carried out with respect to both the two types of CD- and DVD-standardized disks. For instance, as shown in FIGS. 20 and 21, an optical head device has been proposed that includes an integrated semiconductor laser device in which two semiconductor laser elements with different emission wavelengths from each other are integrated (see, for example, JP11 (1999)-186651A). The following description outlines the above configuration.
FIG. 20 is a perspective view showing a conventional integrated semiconductor laser device. As shown in FIG. 20, in the integrated semiconductor laser device, an AlGaAs semiconductor laser 131 having an emission wavelength in the 700 nm band (for example, 780 nm) and an AlGaInP semiconductor laser 132 having an emission wavelength in the 600 nm band (for example, 650 nm) are integrated on an n-type GaAs substrate 101 while being separate from each other. In order to satisfy the functional needs of a laser, the AlGaAs semiconductor laser 131 is composed of a plurality of layers formed in accordance with known techniques, such as an n-type GaAs buffer layer 111, an n-type AlGaAs cladding layer 112, an active layer 113, a p-type AlGaAs cladding layer 114, a p-type GaAs cap layer 115, and an n-type GaAs current constriction layer 116. Likewise, the AlGaInP semiconductor laser 132 is composed of a plurality of layers such as an n-type GaAs buffer layer 121, an n-type AlGaInP cladding layer 122, an active later 123, a p-type AlGaInP cladding layer 124, a p-type GaInP intermediate layer 125, a p-type GaAs cap layer 126, and an n-type GaAs current constriction layer 127. Further, in this integrated semiconductor laser device, p-side electrodes are separated so that the AlGaAs semiconductor laser 131 and the AlGaInP semiconductor laser 132 can be driven independently. More specifically, the AlGaAs semiconductor laser 131 can be driven upon passing a current between a p-side electrode 117 and an n-side electrode 129; and the AlGaInP semiconductor laser 132 can be driven upon passing a current between a p-side electrode 128 and the n-side electrode 129. Incidentally, the AlGaAs semiconductor laser 131 and the AlGaInP semiconductor laser 132 are placed on a package base 130 with heat sinks 133 and 134 interposed therebetween, respectively.
FIG. 21 is a layout drawing showing a configuration of an optical disk device for reproducing CDs and DVDs including the integrated semiconductor laser device in FIG. 20. The integrated semiconductor laser device shown in FIG. 20 is used as a semiconductor laser 201 shown in FIG. 21. As shown in FIG. 21, a light beam L emitted from the semiconductor laser 201 is converted to parallel light by a collimator lens 202, and then passes through a beam splitter 203. Subsequently, the light beam L passes through a ¼ wave plate 204 while the degree of its polarization is adjusted thereby. The light beam L whose polarization has been adjusted is focused by an objective lens 205 and is incident on an optical disk 209. Then, signal light L′ reflected from the optical disk 209 passes through the objective lens 205 and the ¼ wave plate 204, and then is reflected by the beam splitter 203. Subsequently, the signal light L′ goes through a detecting lens 206 and then enters a signal-light detecting photodetector 207. The signal light L′ that entered the signal-light detecting photodetector 207 is converted to an electric signal therein and this electric signal is transmitted to a signal-light-reproducing circuit 208. The information written on the optical disk 209 is thus reproduced.
However, when using the conventional integrated semiconductor laser device described above, the optical axes of the AlGaAs semiconductor laser 131 and the AlGaInP semiconductor laser 132 actually are separated by a spacing between their beam emission points. Because of this, as shown in FIG. 22, positions of spots 301 and 302 of laser beams emitted from the AlGaAs semiconductor laser 131 and the AlGaInP semiconductor laser 132, respectively, are separated from each other by a distance of (a spacing between beam emission points)/(optical magnification). Thus, for instance, when the optical axis of a light beam emitted from the AlGaAs semiconductor laser 131 is adjusted to coincide with a center axis of the objective lens 205, the optical axis of a light beam emitted from the AlGaInP semiconductor laser 132 is offset with respect to the center axis of the objective lens 205. As a result, there arises a problem that as the objective lens 205 shifts in a radial direction in the optical disk 209, the variation in a tracking-error signal amount is not balanced, as shown with a characteristic curve G in FIG. 23. Furthermore, when the tracking-error signal amount varies as shown in FIG. 23, particularly, the signal amount is degraded sharply with respect to a shift towards a negative direction. Accordingly, the tracking servo operation is destabilized. Incidentally, numeral 303 in FIG. 22 indicates information recording pits provided on the optical disk 209.